This invention is related to methods of controlling viscosity in the practice of fracturing subterranean formations, particularly as is done in the oil and gas industry.
Fluids exhibit a measurable property known as viscosity, a term which may be broadly defined as the internal friction or molecular attraction of a given material which manifests itself in resistance to flow. It is measured in liquids by standard test procedures and is often expressed in centipoises at a specified temperature and optionally other conditions. The viscosity of a liquid is an indication of a number of behavior patterns of the liquid at a given temperature including pumping characteristics, rate of flow, wetting properties and a tendency or capacity to suspend an insoluble particulate material therein.
A particular need exists for control of viscosity in the practice of fracturing subterranean formations such as is done in the oil and gas industry. A fracturing fluid (usually a liquid, but occasionally some gas will be present), having granular particulate material suspended or slurried therein, is injected down a well to penetrate the formation and is forced back into the formation. Typically, the height of the hydraulic head of liquid is used to provide added pressure and hence this process is sometimes called hydraulic fracturing. The injection of this fluid causes the formation to crack or fracture, thereby permitting oil or gas in the formation to move more easily to the producing well. Fracturing usually employs oil, water or an emulsion thereof which has suspended therein particles which are usually insoluble in the liquid and the fluids of the formation. At least a portion of the particles generally lodge in the fractures created in the formation, thus propping the fractures open. This keeps the fractures open when the pressure is released and the well put back into production.
One of the problems of fracturing fluids is the tendency of the particulate proppants to settle out of the fluid prematurely and thus not be available to prop the fractures open at distances removed from the injection well. One approach to solving this problem is to thicken or gel the fracturing fluid so that it holds the proppants in place. This can result in an additional problem since it is difficult to remove the thickened fracturing fluid from the formation.
It is known to use "breakers" with fracturing fluids. The breakers reduce the viscosity of the fluid at the appropriate time so that the proppant is available to hold open the fractures in the formation. The breaker then permits the fluid to thin enough to be easily removed from the formation while leaving the proppant behind. For example, U.S. Pat. No. 3,818,991 to Nimerick, dated June 25, 1974, teaches the use of a fracturing composition containing a water soluble hydrazine or hydroxylamine to decrease the viscosity at a given time. U.S. Pat. No. 4,552,674 to Brown et al., dated Nov. 12, 1985, teaches the use of peroxygen compounds capable of generating free radicals at the appropriate time to decrease the composition viscosity. Other references teach the use of persulfates.
Certain problems associated with the use of breakers to decrease viscosity of fracturing fluids after the fluids have delivered the proppants to the formation remain. The breaker must be stable under the conditions at which the fracturing fluid is delivered to the formation and must quickly become available once the proppant is in place so that the fracturing fluid thins enough to be removed. There remains a need for breaker fluids to meet these criteria under the various environmental conditions existing in subterranean formations where oil and gas are found.